The long-term objective of the project is, by describing the neural organization of masticatory muscles, to further understanding of normal oral function and its developmental mechanisms. The pig masticatory apparatus provides an excellent model for the study of architecturally complex jaw muscles, such as also occur in humans. The specific aims are directed at solving four central problems. First, axons which supply distant portions of the masseter may or may not be located adjacent to each other. To test the hypothesis of adjacency, horseradish peroxidase (HRP) will be injected into different portions of the muscle to reveal the spatial relations of the axons. Second, the masseter contraction pattern differs in infant and adult pigs. The difference may be caused by changes in the innervation of the muscle. To test this hypothesis, maps will be made for (a) motor unit territories within the masseter (using glycogen depletion of stimulated muscle fibers), and (b) locations of masseteric cell bodies within the trigeminal motor nucleus (via retrograde labeling with HRP). The results will be compared for pigs of different ages. Third, it is not known why different compartments of the masseter develop functional independence at different postnatal ages. This inequality may be a function of a variable maturation rate in different parts of the muscle. To test for variations in maturation rate, different parts of the masseter will be examined for developmental differences in fiber size and fiber type. Alternatively, the inequality in time of achieving functional independence may be related to changes in fiber orientation, which will be tested by quantifying the architecture of muscles from animals of different ages. Fourth, too few data on overall masticatory development are available to allow the details of masseter function to be considered in the context of the oral apparatus as a whole. Therefore, electromyographic studies will be carried out on (a) the hyoid and tongue muscles during suckling, and (b) the pinnation compartments of the temporalis and medial pterygoid during mastication. Information gained from this project will be clinically useful in choosing portions of masticatory muscles which can be effectively used as substitutes for paralyzed facial muscles, in understanding why certain parts of jaw muscles are prone to spasm, and in the management of traumatic injuries to masticatory muscle nerves.